1. The Field of the Invention
This invention relates to systems and methods for noninvasively measuring one or more biologic constituent values. More particularly, the present invention relates to noninvasive spectrophotometric systems and methods for quantitatively and continuously monitoring the hematocrit and other blood parameters of a subject.
2. The Prior Art
Modern medical practice utilizes a number of procedures and indicators to assess a patient's condition. One of these indicators is the patient's hematocrit. Hematocrit (often abbreviated as Hct) is the volume, expressed as a percentage, of the patient's blood which is occupied by red corpuscles (commonly referred to as red blood cells).
Human blood consists principally of liquid plasma (which is comprised of over 90% water with more than 100 other constituents such as proteins, lipids, salts, etc.) and three different corpuscles. The three corpuscles found in blood are red corpuscles, white corpuscles, and platelets.
The chief function of red corpuscles is to carry oxygen from the lungs to the body tissues and carbon dioxide from the tissues to the lungs. This critical life supporting function is made possible by hemoglobin which is the principal active constituent of red corpuscles. In the lungs, hemoglobin rapidly absorbs oxygen to form oxyhemoglobin which gives it a bright scarlet color. As the red corpuscles travel to the tissues, the oxyhemoglobin releases oxygen, i.e., is reduced, and the hemoglobin turns a dark red color.
The oxygen transportation functions of the body rely essentially entirely on the presence of hemoglobin in the red corpuscles. Red corpuscles greatly outnumber other corpuscles being about 700 times greater than the number of white corpuscles in a healthy human subject.
Medical professionals routinely desire to know the hematocrit of a patient. In order to determine hematocrit using any of the techniques available to date, it is necessary to draw a sample of blood by puncturing a vein or invading a capillary. Then, using a widely accepted technique, the sample of blood is subjected to a high speed centrifuge treatment for several minutes (e.g., 7 or more minutes). The centrifuging process, if properly carried out, separates the corpuscles into a packed mass. The volume occupied by the packed corpuscles, expressed as a percentage of the total volume of the plasma/corpuscle combination, is taken as the hematocrit.
It will be appreciated that the centrifuge process provides a hematocrit value which includes all corpuscles, not just red corpuscles. Nevertheless, the vastly greater numbers of red corpuscles in a healthy subject allows the hematocrit value obtained by the centrifuge process to be clinically usable in such healthy subjects. Nevertheless, in subjects with low hematocrit or dramatically high white corpuscle content, it may be desirable to diminish the effect of the non-red corpuscles when obtaining an hematocrit value.
There have, been various techniques and devices introduced which have automated and increased the precision of obtaining a hematocrit value. Nevertheless, all the previously available techniques have one or more drawbacks.
Specifically, the previously available techniques all require that a sample of blood be withdrawn from the patient for in vitro analysis. Any invasion of the subject to obtain blood is accompanied by the problems of inconvenience, stress, and discomfort imposed upon the subject and also the risks which are always present when the body is invaded. Drawing blood also creates certain contamination risks to the paramedical professional. Moreover, even in a setting where obtaining a blood sample does not impose any additional problems, e.g., during surgery, the previously available techniques require a delay between the time that the sample is drawn and the hematocrit value is obtained. Still further, none of the previously available techniques allow continuous monitoring of a subject's hematocrit, as might be desirable during some surgical procedures or intensive care treatment, but require the periodic withdrawal and processing of blood samples.
In view of the drawbacks inherent in the available art dealing with invasive hematocrit determinations, it would be an advance in the art to noninvasively and quantitatively determine a subject's hematocrit value. It would also be an advance in the art to provide a system and method for noninvasive hematocrit monitoring which can be applied to a plurality of body parts and which utilizes electromagnetic emissions as an hematocrit information carrier. It would be another advance in the art to provide a system and method which can provide both immediate and continuous hematocrit information for a subject. It would be yet another advance to provide repeatable and reliable systems for noninvasive monitoring of a subject's hematocrit. It would be still another advance in the art to noninvasively and accurately determine a subject's blood oxygen saturation while accounting for the patient's low or varying hematocrit and/or under conditions of low perfusion.